The invention relates to an apparatus and a process for the production of tubular bodies, in particular for packaging tubes, from a foil strip comprising weldable plastic material, the longitudinal edges of which are thermally joined together.
U.S. Pat. No. 3,388,017 discloses an apparatus for the production of tubular bodies from a thermoplastic strip material, wherein the strip material is supported from below by a driven conveyor belt which is of smaller width than the strip material, and is introduced jointly with same into a divided, upwardly open shaping sleeve.
The arrangement also has two driven transportation belts of which one is guided in grooves in a mandrel and the other is arranged in such a way that the overlap location of the tubular strip is guided by the two transportation belts, in clamped relationship and subjected to pressure, during the passage of the strip through the shaping body formed from the shaping sleeve.
The transfer of heat in the seam welding operation is effected by contact heat which is applied to the foil and removed therefrom by heating bodies and downstream-disposed cooling bodies, by way of a transportation belt. It has been found that that known apparatus suffers from the disadvantage that uncontrollable frictional forces occur in the shaping body, and those forces frequently result in relative movements between the transportation belts and a seam which has already been softened, thus resulting in unusable tubular bodies, due to the poor appearance of the seam.
In accordance with the known construction, a transportation belt co-operates both with the welding zone and also with the cooling zone. As the transportation belt contains different amounts of heat, corresponding to the respective zones, in the event of relative movements the quality of welded seams is adversely affected to a clearer extent than if a first transportation belt were to cooperate only with the welding zone, and a second transportation belt were to co-operate with the cooling zone. Moreover a one-piece upper transportation belt is subjected to a higher thermal loading than a two-piece belt, and that is considered to be a disadvantage in regard to synchronous movement as between the upper and lower transportation belts.
The other deficiencies from which the construction disclosed in U.S. Pat. No. 3,388,017, in conjunction with technical limitations which, in the view of the men skilled in the art, result from the one-piece upper transportation belts, have resulted in two directions of development in regard to apparatuses for the purpose with which we are concerned here.
Swiss patent specification No. 614 660 discloses an installation for the production of shapable tubular bodies from thermoplastic foil strip, wherein the foil strip may be a metal-plastic composite foil comprising at least three layers, which is referred to as a monofoil. Similarly to the construction disclosed in U.S. Pat. No. 3,388,017, the tubular body is formed in a stationary shaping arrangement comprising a shaping sleeve with an intake funnel portion and a mandrel which extends through the shaping sleeve and the intake funnel portion, in a condition of rubbing engagement of the foil strip with the components of the shaping arrangement. The tube which is formed in the shaping arrangement is moved in the axial direction of the mandrel by means of an upper driven transportation belt and a lower transportation belt which runs freely in the mandrel. The upper driven transportation belt co-operates with a heating and pressing means which heat and press the overlapping edges of the foil while cooling is effected directly, that is to say without using a cooling belt.
In the case of production of tubular bodies from composite foils, the heating effect is preferably to be produced by high-frequency heating, while in relation to monofoils, heating is preferably effected by ultrasound.
As a shaping arrangement is provided, frictional forces occur in the same way as in the shaping apparatus disclosed in U.S. Pat. No. 3,388,017, and those frictional forces result in relative movements between the seam and the transportation belt, with the known disadvantages. By virtue of its relatively uniform heat content, the one-piece upper transportation belt which only co-operates with the heating and pressing means somewhat alleviates the disadvantages of the apparatus disclosed in U.S. Pat. No. 3,388,017, but it does not completely eliminate them.
An apparatus for the production of tubular bodies, which represents another direction of development, is known from European patent application No. 0 264 663. The apparatus is supplied with a foil strip which includes an aluminum layer embedded into at least two plastic layers. In the apparatus the foil strip is rolled around a mandrel to form a tube with overlapping ends by means of a shaping belt co-operating with guide elements. The stationary shaping devices which are known from the described state of the art are replaced in this system by a moving shaping belt on which the foil lies during the shaping operation, free from any movement between the shaping belt and the foil, and thus in a friction-free condition.
The overlapping ends pass between a lower driven transportation belt which runs on the mandrel and two upper driven transportation belts which are arranged one after the other, wherein the welding operation is effected by heat and pressure by means of the first transportation belt and cooling of the welded seam is effected by the second downstream-disposed transportation belt.
Heating of the plastic layers, which is required to produce the welding effect, is effected by the metal layer being inductively heated by high frequency during the operation in which the edges are pressed together, so that the welding heat is produced directly in the foil. Due to the use of a two-piece upper transportation belt, the welded seam can also be shaped during the cooling operation under the action of external forces, without the higher level of thermal loading with the known effects thereof in regard to synchronous movement having to be tolerated, as in the case of a one-piece transportation belt which passes beneath the welding and cooling means.
It has been found that that design configuration suffers from the disadvantage that it cannot be used to weld monofoils with the required level of quality; the term monofoils also includes multi-layer plastic foils in which for example a gas-tight plastic ply or layer is embedded between two non-gas-tight plies or layers, for example of polyethylene. The required level of quality is not achieved by heating of the transportation belts by inductive heating means with heat then being given off to the monofoil, or by the use of ultrasound for foil heating purposes.